In general, semiconductor memories may be divided into three classes: random-access memories (RAM's), read-only memories (ROM's), and programmable read-only memories (PROM's). RAM's are used to store large volumes of data which must be read and written at high speeds. ROM's may not be electrically written; they store data which may be read at high speeds but not changed by the system during operation. PROM's may be erased and reprogrammed, but only at very slow rates, so that reprogramming is not feasible during system operation. In effect, PROM's may be regarded as ROM's which may be programmed a few times by the user.
There are two important operating characteristics which describe semiconductor memories: (i) volatility, and (ii) writability. Volatility refers to the time period over which a memory will store data. A memory is said to be nonvolatile if the data is retained for a useful period of time after power is removed. Both ROM's and PROM's are nonvolatile. Writability refers to the ability of the memory to be written with new data at high speeds by the system during operation, i.e., the ability to store data which is the result of calculations performed by the system or new data to be processed by the system. In this sense, RAM's are writable, but ROM's are not.
Thus, RAM's offer writability, but not nonvolatility. ROM's are nonvolatile, but also nonwritable.
RAM's can be classified as either static RAM's or dynamic RAM's. Dynamic RAM's, or DRAM's, are widely used in modern digital computing systems for high-speed data storage and retrieval. In a DRAM, the data storage is said to be "dynamic," because data is retained only for a relatively short time, typically on the order of a few seconds at room temperature. The basic DRAM cell consists of an access transistor and a storage capacitor. Data is written into the cell by turning on the access transistor so that the potential applied to the bit line is connected directly to the storage capacitor. When the access transistor is turned off, the storage capacitor remains charged to the potential of the bit line until unwanted leakage currents slowly discharge it. See articles by Sander et al. entitled High Density Memories, Institute for Electronic and Electrical Engineers International (IEEE) Solid-State Circuits Conference, Digest of Technical Papers, pp. 182-83, 1976, and by Antipov entitled Proposed Process Modifications for Dynamic Bipolar Memory to Reduce Emitter-Base Leakage Current, IEEE Transactions on Electron Devices, Vol. ED-27, No. 8, pp. 1649-54, 1980.
The "storage time" of a DRAM cell is an important parameter of the cell. Since the data decays, the memory controller must periodically read and rewrite the contents of each cell in the array, a process referred to as "refresh." Typical refresh rates in modern systems are around 1 kilohertz (kHz), requiring storage times greater than about 20 milliseconds (ms). In spite of the system overhead imposed by the refresh requirement, DRAM's are widely used because of their simplicity and small cell size. For DRAM configurations described above, see articles by Quinn et al. entitled High Density Memories, IEEE International Solid-State Conference, 1978, and by Sunami entitled Cell Structures for Future DRAM's, IEEE IEDM, pp. 694-97, 1985. DRAM's also have the added advantage of dissipating almost no static power while storing data.
The advantages of DRAM's are best appreciated in comparison to programmable read-only memories (PROM's). PROM's are nonvolatile, but may not be electrically written during operation, that is, they do not have writability. PROM's typically store data by transferring charge to a region isolated by a large potential barrier. This region typically takes the form of a floating gate surrounded by an insulating barrier of silicon dioxide in a metal-oxide-semiconductor field-effect transistor (MOSFET) geometry. Charge is transferred to and from the floating gate by avalanche injection of electrons over or tunneling through the potential barrier of the oxide at high electric fields. Because the potential barrier is very large, leakage is negligible at normal operating temperatures, and storage is nonvolatile.
Electrical readout of the stored data in a PROM is accomplished by detecting the current in the underlying MOSFET; in effect, the floating gate shifts the threshold voltage of the MOSFET, altering the current. Reading is therefore very fast. There are a great variety of similar structures, all operating on a similar basic principle. See text in publication by Sze entitled Nonvolatile Memory Devices, Physics of Semiconductor Devices, pp. 496-506, John Wiley & Sons, Inc., New York, 1981. For example, some devices store charge in deep traps within a special dualdielectric insulator, rather than on an electrode. Examples are the MNOS (for metal-nitride-oxide-semiconductor) memory and the doped-interface dual-dielectric storage cell (where charge is stored on a sub-monolayer of metal atoms deposited at the dielectric interface).
Unfortunately, all PROM's suffer from two important drawbacks. First, writing data into the memory is very slow (typically one-tenth of a millisecond for each cell). Second, all these devices are subject to a "wearout" mechanism, in which the cell's operation degrades after about 10.sup.6 -10.sup.7 write operations. If the memory is only reprogrammed a few times, this wearout mechanism does not present any difficulties. Any attempt, however, to use this type of cell as a read/write memory would quickly exceed the wearout limit of the dielectric, since 10.sup.6 writing operations may typically take place in just a few seconds in a high-speed computing system.
In summary, there is no semiconductor memory known to the art that is both nonvolatile and writable during operation.